Chemistry
Chapter 12 Stoichiometry
Notes Outline
Percent Composition
Gravimetric analysis and volumetric analysis are experimental procedures based on the
measurement of mass for solids and liquids, respectively.
Percent composition from experimental data: The percent by mass of any element in a
compound can be found by dividing the mass of the element by the mass of the compound
and multiplying by 100.
percent by mass (element) =
mass of the element x 100
mass of the compound
The percent by mass of each element in a compound is the percent composition of a
compound.
Percent composition from the chemical formula: The percent composition of a compound
can be determined from its chemical formula. To do this, assume you have exactly one mole
of the compound and use the chemical formula to calculate the compound’s molar mass.
Then determine the mass of each element in a mole of the compound by multiplying the
element’s molar mass by its subscript.
percent by mass = mass of element in 1 mol of compound x 100
molar mass of compound
Example Problem 1 Calculating Percent Composition: Sodium hydrogen carbonate (NaHCO3),
also known as baking soda, is an effective ingredient in some antacids used for the relief
of indigestion. Determine the percent composition of NaHCO3.
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Practice Problems
1. What is the percent composition of phosphoric acid (H3PO4)?
2. Which has the larger percent by mass of sulfur, H2SO4 or H2S2O8?
3. Calcium chloride (CaCl2) is sometimes used as a de-icer. Calculate the percent by
mass of each element in CaCl2.
4. Challenge Sodium sulfate is used in the manufacture of detergents.
a. Identify each of the component elements in sodium sulfate, and write the
compound’s chemical formula.
b. Identify the compound as ionic or covalent.
c. Calculate the percent by mass of each element in sodium sulfate.
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Empirical Formula
When a compound’s percent composition is known, its formula can be calculated. First,
determine the smallest whole-number ratio of the moles of the elements in the compound.
This ratio gives the subscripts in the empirical formula. The empirical formula for a
compound is the formula with the smallest whole-number mole ratio of the elements. The
empirical formula may or may not be the same as the actual molecular formula. If the two
formulas are different, the molecular formula will always be a simple multiple of the
empirical formula.
Example Problem 2 Empirical Formula from Percent Composition: Methyl acetate is a
solvent commonly used in some paints, inks, and adhesives. Determine the empirical formula
for methyl acetate, which has the following chemical analysis: 48.64% carbon, 8.16%
hydrogen, and 43.20%oxygen.
Practice Problems
5. A blue solid has the following percent composition: 63.16% O and 36.84% N. What is
the empirical formula for this solid?
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6. Determine the empirical formula for a compound that contains 35.98% aluminum and
64.02% sulfur.
7. Propane is a hydrocarbon, a compound composed only of carbon and hydrogen. It is
81.82% carbon and 18.18% hydrogen. What is the empirical formula?
8. Challenge Aspirin is the world’s most-often used medication. The chemical analysis
of aspirin indicates that the molecule 60.00% carbon, 4.44% hydrogen, and 35.56%
oxygen. Determine the empirical formula for aspirin.
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Molecular Formula
The molecular formula specifies the actual number of atoms of each element in one
molecule or formula unit of the substance.
molecular formula = (empirical formula)n


A molecular formula can be represented as the empirical formula multiplied by an
integer n.
The integer is the factor by which the subscripts in the empirical formula must be
multiplied to obtain the molecular formula.
Example Problem 3 Determining a Molecular Formula: Succinic acid is a substance produced
by lichens. Chemical analysis indicates it is composed of 40.68% carbon, 5.08% hydrogen,
and 54.24% oxygen and has a molar mass of 118.1 g/mol. Determine the empirical and
molecular formulas for succinic acid.
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Example Problem 4 Calculating an Empirical Formula from Mass Data: The mineral ilmenite
is usually mined and processed for titanium, a strong, light, and flexible metal. A sample of
ilmenite contains 5.41 g of iron, 4.64 g of titanium, and 4.65 g of oxygen. Determine the
empirical formula for ilmenite.
Practice Problems
9. A compound was found to contain 49.98 g of carbon and 10.47 g of hydrogen. The
molar mass of the compound is 58.12 g/mol. Determine the molecular formula.
10. A colorless liquid composed of 46.68% nitrogen and 53.32% oxygen has a molar mass
of 60.01 g/mol. What is the molecular formula?
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11. When an oxide of potassium is decomposed, 19.55 g of K and 4.00 g of O are
obtained. What is the empirical formula of the compound?
12. Challenge Analysis of a chemical in photographic developing fluid yielded 65.45% C,
29.09%O, and 5.45% H. If the chemical’s molar mass is 110.0 g/mol. What is its
molecular formula?
13. Challenge Analysis of the pain reliever morphine yielded the following data: 17.900 g
carbon, 1.680 g hydrogen, 4.225 g oxygen, 1.228 g nitrogen. Determine the empirical
formula of morphine.
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